The present invention relates to conveyors of the type wherein a load, either as a separate article or contained on a pallet, is moved by gravity down a rollerway conveyor, which comprises a plurality of rollers having parallel, coplanar and horizontal axes. Specifically, the present invention relates to an improvement in the roller construction for such a rollerway gravity conveyor.
A rollerway gravity conveyor of the type that may employ the improved roller of the present invention is shown in the patent to Andrew T. Kornylak issued May 13, 1969 as U.S. Pat. No. 3,443,674. This conveyor has a plurality of rollers with annular elastomeric tires mounted on a hub that in turn is mounted by a roller bearing on an axle. The anti-friction bearing is used to reduce the rolling resistance of the roller and to try and keep the rolling resistance uniform over a range of different loads. The compressible tire is desirable to permit defects or other height variations in the pallet bottom surface or the article itself to ride "through" the tire instead of having to climb over it. This results in a reduced slope requirement for gravity rollerways and a smoother pallet or article movement for all types of roller conveyors.
Conveyor rollers with an annular resilient or elastomeric tire may be constructed so as to have a substantial amount of hysteresis, which is the ability of the tire to absorb energy from the load to control the speed of the movement of a load down a gravity conveyor, particularly, to absorb more energy with higher speed to either slow down a fast moving load or prevent a load from building up speed as it moves down a gravity rollerway conveyor. An example of such high hysteresis is found in the U.S. patent to Andrew T. Kornylak issued Nov. 23, 1971 as U.S. Pat. No. 3,621,960. Some resilient or elastomeric materials usable as annular tires on such rollers have no appreciable hysteresis.
Rolling resistance of such a conveyor wheel is defined as the pull required to move a load supported by such a roller, as measured at substantially zero speed to eliminate the effect of hysteresis. Thus, hysteresis will not affect the rolling resistance value of tire materials, which rolling resistance value varies greatly for different elastomeric materials used for roller tires. It is theorized, but not known, that the rolling resistance is caused by internal molecular friction. Although it cannot be adequately explained, it is easily measured on an experimental basis. While hysteresis is a desirable property for speed control, rolling resistance is most always an undesirable property, but a necessary evil of having a resilient tire. Rolling resistance will increase, in general, for an increase in wheel load, which is undesirable for a gravity conveyor.
If the inclination of a particular gravity conveyor is set up so that a load under restraint will reliably start moving when the restraint is removed (that is the inclination is sufficiently great) while at the same time the load will not gain excess speed over the length of its movement (that is, the inclination is sufficiently small), with a satisfactory compromise for these conflicting characteristics, the conveyor may be totally unsuited for a load of substantially different weight per wheel.